Note to readers: You’re going to yawn a lot when you read this article. Just a warning.

So why does that happen? Why, when we see someone sitting across from us yawn on public transit, do we do it too? Even when we’re forcibly trying to suppress it, there it is: a big ol’ annoying yawn. We even yawn sometimes after our dog does.

The definitive answer is elusive — scientists still aren’t 100 per cent certain why humans and their closest relatives (chimps, for example) have “contagious” yawns. An often-used explanation for why we yawn goes like this: when we open wide, we suck in oxygen-rich air. The oxygen then enters our bloodstream and helps to wake us up.

Sorry to mislead you, but that explanation is a myth; so far, there’s no evidence that yawning impacts levels of oxygen in the bloodstream, blood pressure or heart rate. So now what?

Over the past decade or so, scientists think they’ve honed in on why we yawn (it seems like there are more pressing areas of research), and it’s all because of our brain temperature. Yawning is kind of like a radiator — we stretch our jaws, open our mouths and take a big, long inhalation, followed by a brief exhalation. This causes the air reaching our brains to increase, which in turn cools them down when they’re hot.

Yawning as a thermoregulatory system mechanism could explain why we yawn most often when it’s almost bedtime or right as we wake up. “Before we fall asleep, our brain and body temperatures are at their highest point during the course of our circadian rhythm,” Andrew Gallup, a psychology professor, says. “Once we wake up, our brain and body temperatures are rising more rapidly than at any other point during the day.”

OK, fine. Yawning cools our brains. But that still doesn’t explain why we yawn when others yawn. Or does it?

Humans have some primal, ingrained drive inside us to succeed and stay alive. So it makes sense that, if one of your group’s members (meaning someone you happen to be chatting with, or a family member at dinner) has an overheating brain and yawns, then a subconscious signal will trigger you to yawn as well. Your group is only as weak as your weakest member, correct? Since that person is in close proximity, your body registers any threat against them as a threat against you, thus prompting you to yawn. This unintentional copycat behaviour could improve your alertness, thereby improving your entire group’s chances of survival.

This hypothesis is the current leader in the struggle to figure out why we yawn in the first place.

(We hope you’re not one of those people who doesn’t yawn when someone close to them yawns. A recent study indicated that people who don’t may have a tendency to be psychopathic and completely void of empathy. Yikes.)

Again, we’re sorry about all the yawning you’ve probably done in the last five minutes. Not like we (or you) could help it.